The invention concerns a process for preparing an anion-exchanging mineral and the use of such minerals which reversibly bind (exchange) anions such as NO3xe2x88x92, among others, as improvements for fertilizers and soil, or for purifying and treatment of water, especially to remove nitrate.
In a broader sense, the invention concerns the intentional addition and removal of nitrates.
IS Optimal nutrition of crop plants, both in the open air and in the greenhouse, requires, among other things, an adequate nitrogen supply timed to match the plant growth. As a general guideline, one can assume that about 200 kg N/hectare is needed during one vegetation phase, with plants having different needs, depending on the species and variety and on their stage of development.
Supplying nitrogen to plants in the proper amount at the right time is not simple, for various reasons. Nitrogen can be made available in the form of ammonium ions (NH4+) or nitrate ions (NO3xe2x88x92). In the soil, there is a complex equilibrium between the various forms of bound nitrogen. There are microorganisms in the soilxe2x80x94in various proportionsxe2x80x94which can convert ammonia into nitrate.
Because the soils on which we grow crops do not have much anion-exchange ability, though, the nitrate is easily washed out into surface water and ground water. Nitrification inhibitors are often used at times to inhibit the soil microorganisms so as to avoid excessive conversion of ammonium into nitrate.
Washing out of cations, by comparison, is of secondary importance, because they can be bound to the exchange sites of the clay minerals normally present in the soil. Therefore ammonium ions and the other cations important for plant culture, such as potassium, magnesium, and calcium, can be held well enough in our crop soils. The extremely sandy soils with very low clay content are exceptions, where cation washout is also a problem.
Various ways are known at the state of the art by which useful nitrogen can be made available to plants for long periods. Known fertilizers and soil improvers with depot action, for instance, work with fertilizer mixtures from which nitrogen is supposed to be released at different times in a vegetation period.
For example, a fertilizer with long-term action and programmed nutrient release for providing the nutrient requirements of a plant during a culture period, in the form of a mixture of an initial release, a long-term release, and a final release, is known from DE 33 21 053 C2. The long-term releaser and the final releaser comprise particles of fertilizer of certain particle sizes with coatings which prevent immediate release of nitrogen.
Furthermore, soil substrates are also known which can be used directly for growing cropsxe2x80x94in greenhouses, for instancexe2x80x94or as soil improvers. They are reported to consist for the most part of neutral porous materials such as zeolites and the like, and their physical adsorption and filtering actions are utilized.
The known fertilizing and soil-improving agents also have the disadvantage that they do not release nutrients as they are required by the plants. Rather, the release is a result of the action of soil factors (temperature, water, microorganisms). It follows that nutrients which are released because of soil factors, with simultaneous low requirement of the plants for nutrients, are potentially exposed to washout and can cause pollution.
Thus the invention is based on the problem of finding an environmentally acceptable anion exchanger with particularly good exchange capacity for nitrate ions.
With respect to a fertilizer improver and soil improver, the objective is to find such an agent that has a buffering action on the nitrate content in the soil solution. Thus it can provide the nitrogen requirement of the plants through continuous release of nitrate as needed. On the other hand, it can bind again any excess nitrate in the soil and in the ground water flowing through it. Thus the agent can simultaneously supply crop soils with nitrogen in the form of nitrate.
With respect to an agent for purifying and treating water, the objective is to remove nitrate efficiently and economically from drinking water or waste water.
These objectives are attained through the process according to the invention for preparing an anion exchanging mineral and the use of anion-exchanging minerals which exchange NO3xe2x88x92, among others, reversibly, as fertilizer and soil improvers and for purifying and treating water.
Experiments by the inventor have shown that certain mineral double salts can be used well as anion exchangers in the sense of this invention. A special precipitation process is recommended here to prepare such anion-exchanging minerals as are suitable to attain the objective.
The process according to the invention comprises
coprecipitation from a highly carbonate-free aqueous alkaline solution
of at least one metal salt from the group:
Ca2+, Mg2+, Fe2+, Ni2+, Zn2+, Co2+, Cu2+, Mn2+, Lixe2x88x92, nitrate, sulfate, chloride or hydroxide
and at least one metal salt from the group Al3+, Fe3+, Cr3+, Mn3+, nitrate, sulfate, chloride, or hydroxide, with the precipitation reaction controlled over an extended period;
separation of the precipitated product, and
heat-treating the precipitated product, i.e., carrying out a thermal treatment at up to 350xc2x0 C., preferably up to 250xc2x0 C.
The carbonate content during the precipitation should be as low as possible, as carbonate is not exchangeably bonded in the material, so that anion exchange is severely reduced by any carbonate content. On precipitation over an extended period, one gets a well-crystallized laminar double hydroxide (LDH) which, as described below, exhibits good exchange characteristics under soil conditions and which, charged with the appropriate anions, is also well suited for water purification.
The precipitation reaction should take place over a long period. After precipitation, a heat treatment up to 300xc2x0 C., preferably up to 250xc2x0 C., is carried out to improve the crystallinity and exchange behavior.
In a further development of the invention, the precipitated or separated mineral can, after washing and drying, be treated with acid and/or phosphate solution. The post-treatment with biphosphate salts causes coagulation (flocculation of the individual particles). The post-treatment with acid is done for further deliberate influence on the crystallinity.
Preferably the pH of the solution is held constant in the alkaline range during the precipitation, preferably at pH 12xc2x12. Accurate control of the pH also improves the crystallinity of the product. For that reason the precipitation should be monitored, as with a pH-stat.
Potassium hydroxide (KOH) is used preferably as the base to adjust the alkaline medium.
Particularly good results for the purpose of the invention have been achieved if the first group of metal salts comprises magnesium nitrate and the second group of metal salts comprises Al3+ or Fe3xe2x88x92 nitrate. Minerals obtained from these salt combinations are particularly suitable as fertilizer and soil improvers. The exact composition of cations and anions can be made dependent on the particular area of application, i.e., on the nature of the soil and the crop species, as they affect the exchange behavior.
One the other hand, minerals synthesized from Ca2+, Mg2+, sulfate, chloride and hydroxide as salts of the first group and from Al3+, Fe3xe2x88x92, Cr3+, Mn3+, sulfate, chloride, and hydroxide as salts of the second class are preferred for water treatment and purification. These materials are also suitable as soil improvers.
The minerals prepared by the process according to the invention, which exchange NO3xe2x88x92, among others, reversiblyxe2x80x94or corresponding natural or synthetic mineralsxe2x80x94are usable as fertilizers and soil improvers.
These are preferably natural or synthetic mixed-valence basic metal-metal salts and preferably essentially carbonate-free laminar double hydroxides (LDHs) which have exchangeable anions bound in the intermediate layers and which can be represented by the following formulas:
[MII(1xe2x88x92x)MIIIx(OH)2]x+(Anxe2x88x92x/n)xc2x7m H2O
in which
MII is a bivalent metal ion such as Ca, Mg, Fe, Ni, Zn, Co, Cu, Mn, or 2 Li, preferably Ca, Mg or Fe,
MIII is a trivalent metal ion, preferably Al, Fe, Cr or Mn,
Anxe2x88x92 is a n-valent anion bound in the intermediate layer, such as nitrate, sulfate, chloride or hydroxide.
LDHs comprise bivalent metal ions (MII) surrounded by OH31  Replacement of the bivalent metal ions in the lattice by trivalent metal ions (MIII) produces an excess of positive charge, which can be balanced by anions (Anxe2x88x92) in the intermediate layer. Hydrotalcite and pyroaurite have structures like those of the LDHs.
The anions of the minerals obtained according to the invention can be exchanged appropriately for the particular purpose. For use as a fertilizer, as much nitrate as possible is put into the mineral and thus into the soil. That can be done directly in the synthesis, or it can be done later by saturation with a flowing nitrate solution (on a column, for instance), with the other ions being displaced by nitrate.
For use as an agent for purifying and denitrifying water, the mineral should initially contain as little nitrate ion as possible; that is, practically none. Then the anions are preferably chloride ions. One could also consider SO42xe2x88x92.
Corresponding natural LDH-like minerals can also be used according to the invention as fertilizers and soil improvers.
When it is produced as a fertilizer, the mineral, however obtained, is given the highest possible nitrate content. Preferably the fertilizer and soil improver should be almost completely charged with nitrate when they are produced. That is, at least 80% of the exchangeably bound anions in the mineral can be nitrate ions. The complete charging with nitrate ions, with which at least 80% of the anions in the mineral are nitrate ions, corresponds to up to 30% by weight of nitrate anions in the minerals in question, usually between about 10% and 25% by weight. It can also be desirable to add ions other than nitrate to the soil. In this case, the nitrate ion concentration is optionally less than 80 of the anions, depending on what proportion is chosen for the other anions.
In the following culture period, the nitrate is slowly released by ion exchange. As the nitrate anions, like any anions, have a certain affinity with the mineral to be used according to the invention because of the excess of positive charge in the mineral lattice, there is no simple wash-out of the nitrate. Instead, the nitrate content is in an equilibrium between xe2x80x9cmineral-boundxe2x80x9d and xe2x80x9cin solution (in the natural soil water) so that the nitrate concentration is considerably lower than in soils fertilized in the usual manner, but is still enough to meet the requirements of the plants.
The mineral is very stable, and is retained for a long time during the growth phase. Thus the mineral, after an initial phase of nitrate release, be recharged with nitrate derived from fertilization or mineralization. The nitrate can again be released slowly.
Thus the mineral acts as a soil improver by buffering the nitrate content. That is, it takes up nitrate in times of greater NO3xe2x88x92 availability and releases it when there is a nitrate deficiency. In this way, the nitrate concentration in the ground solution is maintained at a low level when the plants need little or no nitrate (that is important especially in fallow times, as in the Fall and Winter). Then the mineral makes the bound nitrate available again for the next crop. Therefore the minerals according to the invention are nitrate ion exchangers, and nitrate buffers under soil conditions.
The mineral obtained is this or other ways can also be given the lowest possible nitrate content when it is produced as a soil improver and nitrate buffer.
Thus, the mineral used according to the invention or in the fertilizer or soil-improvement agent reduces the nitrate wash-out from crop soils into the ground water, making a substantial contribution to environmental protection.
The invention assures a nitrate concentration in the soil solution which will be enough to provide for the absorption rate of the roots in optimal development of the plants, and is substantially lower than when the usual nitrate fertilizers are used. The exchangeably bound nitrate is released slowly and as needed, as described above. The release is controlled by the nitrate requirement of the plants. Thus the invention involves a fertilizer with depot action (a xe2x80x9ctriggered-release fertilizerxe2x80x9d).
The mineral is stable in the soil for a long time. In contrast to anion exchangers found in the soil in some situations, the exchange capacity is quite independent of the pH. That makes the action reliable under crop conditions. The extremely low anion exchange capacity which soil is known to have is based essentially on amorphous and crystalline iron compounds. Their exchange capacity increases as the pH decreases. Thus the iron compounds are significant only in soils with decidedly acid pH, and otherwise ineffective.
The invention makes it possible to apply the amount of nitrogen needed by a crop in a single application when the crop is started, without the plants being damaged by excessive salt concentration in the soil. The nitrate is held in the upper layer of the soil where roots are thickly developed, so that it is completely available for the plants, particularly for plants which do not have deep roots. Therefore the invention can also be applied advantageously in production of sod, so as to provide the thin layer of soil with anion exchange capability. At the final location, the fertilizing nitrogen is protected from being washed out, as could otherwise easily happen with the very shallow-rooted grass. The invention is also particularly suited for improving the soil of intensively used lawn areas, such as golf courses, sports fields, etc., by giving it anion exchange capacity and thus preventing washout of nitrate.
The invention can be applied in substrates and potting soil as a slow-releasing nitrogen fertilizer. For instance, the invention can be used in commercial gardening for open-air crops, such as Calluna or Erica; for container plants or the like; and in the hobby area for flower beds and balcony plants. The invention is also suitable for lawn fertilization as a slow nitrogen releaser. With this invention, the nitrogen requirement is met continuously over a long period without plant damage by overfertilization.
The invention is also well suited for meeting the nitrogen requirement of crops in hydroponic systems over long periods, and to contribute to stabilization of the salt concentration.
The mineral to be used according to the invention, as a fertilizer and soil improver can also be used with other nutrients in multi-ingredient fertilizers, as in combination with an ordinary mixed fertilizer or with various other fertilizer components and/or other additives.
The fertilizer and soil improver can be applied in strips or rows, or at points, to improve the nitrogen efficiency. Loss of gaseous nitrogen (N2, N2O, NO) and nitrate wash-out are minimized.
The fertilizer can also appear in a preparation with seeds, seedlings, or other propagation material. It is particularly suited for coating seeds with a nitrogen-containing shell to assure the initial nutrition of the seedlings.
The fertilizer and soil improver can be prepared and applied, alone or in combination with other fertilizers, in liquid form, such as an emulsion, gel or paste, or in solid form, as a power, granulate or prills.
The material used as a soil improver according to the invention is also suited for improving areas with high potential for nitrate washout, even if it is not charged with nitrate. This is particularly true for areas with a long history of intensive cultivation, such as with vegetables or special crops, or soils with low water-storage capacity. In general, the invention can be used in all soils where nitrate washout must be expected or is to be prevented.
The special properties pointed out here for nitrate also apply in part for sulfate and similar anions.
The application according to the invention for water purification and treatment, especially for removal of nitrate, is a reversal of the processes that have been presented. With a choice of suitable counterions, such as chloride, it is possible to trap nitrate, by exchange for those counterions, from waters which, for example, have flowed through a mineral layer according to the invention. That can be done, for instance, on an ordinary column, or on a larger scale in a tower. The equilibrium must be maintained so that re-release of nitrate is avoided. Therefore the mineral must be regenerated at certain intervals, as is the case with other industrial ion-exchange processes.
The invention is suitable for purification of wastewater and for purification and treatment of drinking water.
At the same time, the mineral exerts a filtering action, so that large particles and suspended materials can be retained.
The mineral to be used for water treatment can be produced relatively economically. It makes a particularly environmentally favorable treatment process possible, as no synthetic ion-exchangers which would present eventual disposal problems need be used.
With this invention, rather, both applications can favorably be combined, as the ion exchange mineral used for water purification is, when in the charged state (charged with nitrate) is itself a soil improver according to the invention and thus is easily disposed of. It can be added to the compost from city composting plants or even used for fertilization in some other place.